Download The Nobel Prize in Physics 1901-2000

Phys. 641
The Nobel Prize in Physics 1901-2000
The Nobel Prize in Physics 1901-2000
by Erik B. Karlsson*
9 February 2000
What Is Physics?
Physics is considered to be the most basic of the natural sciences. It deals with
the fundamental constituents of matter and their interactions as well as the
nature of atoms and the build-up of molecules and condensed matter. It tries to
give unified descriptions of the behavior of matter as well as of radiation,
covering as many types of phenomena as possible. In some of its applications,
it comes close to the classical areas of chemistry, and in others there is a clear
connection to the phenomena traditionally studied by astronomers. Present
trends are even pointing toward a closer approach of some areas of physics and
microbiology.
Although chemistry and astronomy are clearly independent scientific disciplines,
both use physics as a basis in the treatment of their respective problem areas,
concepts and tools. To distinguish what is physics and chemistry in certain
overlapping areas is often difficult. This has been illustrated several times in the
history of the Nobel Prizes. Therefore, a few awards for chemistry will also be
mentioned in the text that follows, particularly when they are closely connected
to the works of the Physics Laureates themselves. As for astronomy, the
situation is different since it has no Nobel Prizes of its own; it has therefore
been natural from the start, to consider discoveries in astrophysics as possible
candidates for Prizes in Physics.
From Classical to Quantum Physics
In 1901, when the first Nobel Prizes were awarded, the classical areas of
physics seemed to rest on a firm basis built by great 19th century physicists
and chemists. Hamilton had formulated a very general description of the
dynamics of rigid bodies as early as the 1830s. Carnot, Joule, Kelvin and Gibbs
had developed thermodynamics to a high degree of perfection during the
second half of the century.
Maxwell's famous equations had been accepted as a general description of
electromagnetic phenomena and had been found to be also applicable to optical
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radiation and the radio waves recently discovered by Hertz.
Everything, including the wave phenomena, seemed to fit quite well into a
picture built on mechanical motion of the constituents of matter manifesting
itself in various macroscopic phenomena. Some observers in the late 19th
century actually expressed the view that, what remained for physicists to do
was only to fill in minor gaps in this seemingly well-established body of
knowledge.
However, it would very soon turn out that this satisfaction with the state of
physics was built on false premises. The turn of the century became a period of
observations of phenomena that were completely unknown up to then, and
radically new ideas on the theoretical basis of physics were formulated. It must
be regarded as a historical coincidence, probably never foreseen by Alfred Nobel
himself, that the Nobel Prize institution happened to be created just in time to
enable the prizes to cover many of the outstanding contributions that opened
new areas of physics in this period.
One of the unexpected phenomena during the last few years of the 19th
century, was the discovery of X-rays by Wilhelm Conrad Röntgen in 1895,
which was awarded the first Nobel Prize in Physics (1901). Another was the
discovery of radioactivity by Antoine Henri Becquerel in 1896, and the
continued study of the nature of this radiation by Marie and Pierre Curie. The
origin of the X-rays was not immediately understood at the time, but it was
realized that they indicated the existence of a hitherto concealed world of
phenomena (although their practical usefulness for medical diagnosis was
evident enough from the beginning). The work on radioactivity by Becquerel
and the Curies was rewarded in 1903 (with one half to Becqurel and the other
half shared by the Curies), and in combination with the additional work by
Ernest Rutherford (who got the Chemistry Prize in 1908) it was understood
that atoms, previously considered as more or less structureless objects, actually
contained a very small but compact nucleus. Some atomic nuclei were found to
be unstable and could emit the α, β or γ radiation observed. This was a
revolutionary insight at the time, and it led in the end, through parallel work in
other areas of physics, to the creation of the first useful picture of the structure
of atoms.
In 1897, Joseph J. Thomson, who worked with rays emanating from the
cathode in partly evacuated discharge tubes, identified the carriers of electric
charge. He showed that these rays consisted of discrete particles, later called
"electrons". He measured a value for the ratio between their mass and
(negative) charge, and found that it was only a very small fraction of that
expected for singly charged atoms. It was soon realized that these lightweight
particles must be the building blocks that, together with the positively charged
nuclei, make up all different kinds of atoms. Thomson received his Prize in
1906. By then, Philipp E.A. von Lenard had already been acknowledged the
year before (1905) for elucidating other interesting properties of the cathodic
rays, such as their ability to penetrate thin metal foils and produce
fluorescence. Soon thereafter (in 1912) Robert A. Millikan made the first
precision measurement of the electron charge with the oil-drop method, which
led to a Physics Prize for him in 1923. Millikan was also rewarded for his works
on the photoelectric effect.
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In the beginning of the century, Maxwell's equations had already existed for
several decades, but many questions remained unanswered: what kind of
medium propagated electromagnetic radiation (including light) and what
carriers of electric charges were responsible for light emission? Albert A.
Michelson had developed an interferometric method, by which distances
between objects could be measured as a number of wavelengths of light (or
fractions thereof). This made comparison of lengths much more exact than
what had been possible before. Many years later, the Bureau International de
Poids et Mesures, Paris (BINP) defined the meter unit in terms of the number of
wavelengths of a particular radiation instead of the meter prototype. Using such
an interferometer, Michelson had also performed a famous experiment,
together with E. W. Morley, from which it could be concluded that the velocity
of light is independent of the relative motion of the light source and the
observer. This fact refuted the earlier assumption of an ether as a medium for
light propagation. Michelson received the Physics Prize in 1907.
The mechanisms for emission of light by carriers of electric charge was studied
by Hendrik A. Lorentz, who was one of the first to apply Maxwell's equations
to electric charges in matter. His theory could also be applied to the radiation
caused by vibrations in atoms and it was in this context that it could be put to
its first crucial test. As early as 1896 Pieter Zeeman, who was looking for
possible effects of electric and magnetic fields on light, made an important
discovery namely, that spectral lines from sodium in a flame were split up into
several components when a strong magnetic field was applied. This
phenomenon could be given a quite detailed interpretation by Lorentz's theory,
as applied to vibrations of the recently identified electrons, and Lorentz and
Zeeman shared the Physics Prize in 1902, i.e. even before Thomson's discovery
was rewarded. Later, Johannes Stark demonstrated the direct effect of electric
fields on the emission of light, by exposing beams of atoms ("anodic rays",
consisting of atoms or molecules) to strong electric fields. He observed a
complicated splitting of spectral lines as well as a Doppler shift depending on
the velocities of the emitters. Stark received the 1919 Physics Prize.
With this background, it became possible to build detailed models for the
atoms, objects that had existed as concepts ever since antiquity but were
considered more or less structureless in classical physics. There existed already,
since the middle of the previous century, a rich empirical material in the form of
characteristic spectral lines emitted in the visible domain by different kinds of
atoms, and to this was added the characteristic X-ray radiation discovered by
Charles G. Barkla (Physics Prize in 1917, awarded in 1918), which after the
clarification of the wave nature of this radiation and its diffraction by Max von
Laue (Physics Prize in 1914), also became an important source of information
on the internal structure of atoms.
Barkla's characteristic X-rays were secondary rays, specific for each element
exposed to radiation from X-ray tubes (but independent of the chemical form of
the samples). Karl Manne G. Siegbahn realized that measuring characteristic
X-ray spectra of all the elements would show systematically how successive
electron shells are added when going from the light elements to the heavier
ones. He designed highly accurate spectrometers for this purpose by which
energy differences between different shells, as well as rules for radiative
transitions between them, could be established. He received the Physics Prize in
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1924 (awarded in 1925). However, it would turn out that a deeper
understanding of the atomic structure required a much further departure from
the habitual concepts of classical physics than anyone could have imagined.
Classical physics assumes continuity in motion as well as in the gain or loss of
energy. Why then, do atoms send out radiations with sharp wavelengths? Here,
a parallel line of development, also with its roots in late 19th century physics,
had given important clues for interpretation. Wilhelm Wien studied the "blackbody" radiation from hot solid bodies (which in contrast to radiation from atoms
in gases, has a continuous distribution of frequencies). Using classical
electrodynamics, he derived an expression for the frequency distribution of this
radiation and the shift of the maximum intensity wavelength, when the
temperature of a black body is changed (the Wien displacement law, useful for
instance in determining the temperature of the sun). He was awarded the
Physics Prize in 1911.
However, Wien could not derive a distribution formula that agreed with
experiments for both short and long wavelengths. The problem remained
unexplained until Max K.E.L. Planck put forward his radically new idea that the
radiated energy could only be emitted in quanta, i.e. portions that had a certain
definite value, larger for the short wavelengths than for the long ones (equal to
a constant h times the frequency for each quantum). This is considered to be
the birth of quantum physics. Wien received the Physics Prize in 1911 and
Planck some years later, in 1918 (awarded in 1919). Important verifications
that light comes in the form of energy quanta came also through Albert
Einstein's interpretation of the photoelectric effect (first observed in 1887 by
Hertz) which also involved extensions of Planck's theories. Einstein received the
Physics Prize for 1921 (awarded in 1922). The prize motivation cited also his
other "services to theoretical physics," which will be referred to in another
context.
Later experiments by James Franck and Gustav L. Hertz demonstrated the
inverse of the photoelectric effect (i.e. that an electron that strikes an atom,
must have a specific minimum energy to produce light quanta of a particular
energy from it) and showed the general validity of Planck's expressions
involving the constant h . Franck and Hertz shared the 1925 prize, awarded in
1926. At about the same time, Arthur H. Compton (who received one-half of
the Physics Prize for 1927) studied the energy loss in X-ray photon scattering
on material particles, and showed that X-ray quanta, whose energies are more
than 10,000 times larger than those of light, also obey the same quantum
rules. The other half was given to Charles T.R. Wilson (see later), whose
device for observing high energy scattering events could be used for verification
of Compton's predictions.
With the concept of energy quantization as a background, the stage was set for
further ventures into the unknown world of microphysics. Like some other wellknown physicists before him, Niels H. D. Bohr worked with a planetary picture
of electrons circulating around the nucleus of an atom. He found that the sharp
spectral lines emitted by the atoms could only be explained if the electrons
were circulating in stationary orbits characterized by a quantized angular
momentum (integer units of Planck's constant h divided by 2 π ) and that the
emitted frequencies ν corresponded to emission of radiation with energy h ν
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equal to the difference between quantized energy states of the electrons. His
suggestion indicated a still more radical departure from classical physics than
Planck's hypothesis. Although it could only explain some of the simplest
features of optical spectra in its original form, it was soon accepted that Bohr's
approach must be a correct starting point, and he received the Physics Prize in
1922.
It turned out that a deeper discussion of the properties of radiation and matter
(until then considered as forming two completely different categories), was
necessary for further progress in the theoretical description of the microworld.
In 1923 Prince Louis-Victor P. R. de Broglie proposed that material particles
may also show wave properties, now that electromagnetic radiation had been
shown to display particle aspects in the form of photons. He developed
mathematical expressions for this dualistic behavior, including what has later
been called the "de Broglie wavelength" of a moving particle. Early experiments
by Clinton J. Davisson had indicated that electrons could actually show
reflection effects similar to that of waves hitting a crystal and these
experiments were now repeated, verifying the associated wavelength predicted
by de Broglie. Somewhat later, George P. Thomson (son of J. J. Thomson)
made much improved experiments on higher energy electrons penetrating thin
metal foils which showed very clear diffraction effects. de Broglie was rewarded
for his theories in 1929 and Davisson and Thomson later shared the 1937
Physics Prize.
What remained was the formulation of a new, consistent theory that would
replace classical mechanics, valid for atomic phenomena and their associated
radiations. The years 1924-1926 was a period of intense development in this
area. Erwin Schrödinger built further on the ideas of de Broglie and wrote a
fundamental paper on "Quantization as an eigenvalue problem" early in 1926.
He created what has been called "wave mechanics". But the year before that,
Werner K. Heisenberg had already started on a mathematically different
approach, called "matrix mechanics", by which he arrived at equivalent results
(as was later shown by Schrödinger). Schrödinger's and Heisenberg's new
quantum mechanics meant a fundamental departure from the intuitive picture
of classical orbits for atomic objects, and implied also that there are natural
limitations on the accuracy by which certain quantities can be measured
simultaneously (Heisenberg's uncertainty relations).
Heisenberg was rewarded by the Physics Prize for 1932 (awarded 1933) for the
development of quantum mechanics, while Schrödinger shared the Prize one
year later (1933) with Paul A.M. Dirac. Schrödinger's and Heisenberg's
quantum mechanics was valid for the relatively low velocities and energies
associated with the "orbital" motion of valence electrons in atoms, but their
equations did not satisfy the requirements set by Einstein's rules for fast
moving particles (to be mentioned later). Dirac constructed a modified
formalism which took into account effects of Einstein's special relativity, and
showed that such a theory not only contained terms corresponding to the
intrinsic spinning of electrons (and therefore explaining their own intrinsic
magnetic moment and the fine structure observed in atomic spectra), but also
predicted the existence of a completely new kind of particles, the so-called
antiparticles with identical masses but opposite charge. The first antiparticle to
be discovered, that of the electron, was observed in 1932 by Carl D. Anderson
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and was given the name "positron" (one-half of the Physics Prize for 1936).
Other important contributions to the development of quantum theory have been
distinguished by Nobel Prizes in later years. Max Born, Heisenberg's supervisor
in the early twenties, made important contributions to its mathematical
formulation and physical interpretation. He received one-half of the Physics
Prize for 1954 for his work on the statistical interpretation of the wave function.
Wolfgang Pauli formulated his exclusion principle (which states that there can
be only one electron in each quantum state) already on the basis of Bohr's old
quantum theory. This principle was later found to be associated with the
symmetry of wave functions for particles of half-integer spins in general,
distinguishing what is now called fermions from the bosonic particles whose
spins are integer multiples of h /2π
π. The exclusion principle has deep
consequences in many areas of physics and Pauli received the Nobel Prize in
Physics in 1945.
The study of electron spins would continue to open up new horizons in physics.
Precision methods for determining the magnetic moments of spinning particles
were developed during the thirties and forties for atoms as well as nuclei (by
Stern, Rabi, Bloch and Purcell, see later sections) and in 1947 they had reached
such a precision, that Polykarp Kusch could state that the magnetic moment
of an electron did not have exactly the value predicted by Dirac, but differed
from it by a small amount. At about the same time, Willis E. Lamb worked on
a similar problem of electron spins interacting with electromagnetic fields, by
studying the fine structure of optical radiation from hydrogen with very high
resolution radio frequency resonance methods. He found that the fine structure
splitting also did not have exactly the Dirac value, but differed from it by a
significant amount. These results stimulated a reconsideration of the basic
concepts behind the application of quantum theory to electromagnetism, a field
that had been started by Dirac, Heisenberg and Pauli but still suffered from
several insufficiencies. Kusch and Lamb were each awarded half the the Physics
Prize in 1955.
In quantum electrodynamics (QED for short), charged particles interact through
the interchange of virtual photons, as described by quantum perturbation
theory. The older versions involved only single photon exchange, but Sin-Itiro
Tomonaga, Julian Schwinger and Richard P. Feynman realized that the
situation is actually much more complicated, since electron-electron scattering
may involve several photon exchanges. A "naked" point charge does not exist in
their picture; it always produces a cloud of virtual particle-antiparticle pairs
around itself, such that its effective magnetic moment is changed and the
Coulomb potential is modified at short distances. Calculations starting from this
picture have reproduced the experimental data by Kusch and Lamb to an
astonishing degree of accuracy and modern QED is now considered to be the
most exact theory in existence. Tomonaga, Schwinger and Feynman shared the
Physics Prize in 1965.
This progress in QED turned out to be of the greatest importance also for the
description of phenomena at higher energies. The notion of pair production from
a "vacuum" state of a quantized field (both as a virtual process and as a real
materialization of particles), is also a central building block in the modern field
theory of strong interactions, quantum chromodynamics (QCD).
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Another basic aspect of quantum mechanics and quantum field theory is the
symmetries of wave functions and fields. The symmetry properties under
exchange of identical particles lie behind Pauli's exclusion principle mentioned
above, but symmetries with respect to spatial transformations have turned out
to play an equally important role. In 1956, Tsung-Dao Lee and Chen Ning
Yang pointed out, that physical interactions may not always be symmetric with
respect to reflection in a mirror (that is, they may be different as seen in a lefthanded and a right-handed coordinate system). This means that the wave
function property called "parity", denoted by the symbol "P", is not conserved
when the system is exposed to such an interaction and the mirror reflection
property may be changed. Lee's and Yang's work was the starting point for an
intense search for such effects and it was shown soon afterwards that the β
decay and the π→µ decay, which are both caused by the so-called "weak
interaction" are not parity-conserving (see more below). Lee and Yang were
jointly awarded the Physics Prize in 1957.
Other symmetries in quantum mechanics are connected with the replacement of
a particle with its antiparticle, called charge conjugation (symbolized by "C"). In
the situations discussed by Lee and Yang it was found that although parity was
not conserved in the radioactive transformations there was still a symmetry in
the sense that particles and antiparticles broke parity in exactly opposite ways
and that therefore the combined operation "C"x"P" still gave results which
preserved symmetry. But it did not last long before James W. Cronin and Val
L. Fitch found a decay mode among the "K mesons" that violated even this
principle, although only to a small extent. Cronin and Fitch made their discovery
in 1964 and were jointly awarded the Physics Prize in 1980. The consequences
of their result (which include questions about the symmetry of natural
processes under reversal of time, called "T") are still discussed today and touch
some of the deepest foundations of theoretical physics, because the "P"x"C"x"T"
symmetry is expected always to hold.
The electromagnetic field is known to have another property, called "gauge
symmetry", which means that the field equations keep their form even if the
electromagnetic potentials are multiplied with certain quantum mechanical
phase factors, or "gauges". It was not self-evident that the "weak" interaction
should have this property, but it was a guiding principle in the work by Sheldon
L. Glashow, Abdus Salam, and Steven Weinberg in the late 1960s, when
they formulated a theory that described the weak and the electromagnetic
interaction on the same basis. They were jointly awarded the Physics Prize in
1979 for this unified description and, in particular, for their prediction of a
particular kind of weak interaction mediated by "neutral currents", which had
been found recently in experiments.
The last Physics Prize (1999) in the 20th century was jointly awarded to
Gerhardus 't Hooft and Martinus J. G. Veltman. They showed the way to
renormalize the "electro-weak" theory, which was necessary to remove terms
that tended to infinity in quantum mechanical calculations (just as QED had
earlier solved a similar problem for the Coulomb interaction). Their work
allowed detailed calculations of weak interaction contributions to particle
interactions in general, proving the utility of theories based on gauge invariance
for all kinds of basic physical interactions.
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Quantum mechanics and its extensions to quantum field theories is one of the
great achievements of the 20th century. This sketch of the route from classical
physics to modern quantum physics, has led us a long way toward a
fundamental and unified description of the different particles and forces in
nature, but much remains to be done and the goal is still far ahead. It still
remains, for instance, to "unify" the electro-weak force with the "strong"
nuclear force and with gravity. But here, it should also be pointed out that the
quantum description of the microworld has another main application: the
calculation of chemical properties of molecular systems (sometimes extended to
biomolecules) and of the structure of condensed matter, branches that have
been distinguished with several prizes, in physics as well as in chemistry.
Microcosmos and Macrocosmos
"From Classical to Quantum Physics", took us on a trip from the phenomena of
the macroscopic world as we meet it in our daily experience, to the quantum
world of atoms, electrons and nuclei. With the atoms as starting point, the
further penetration into the subatomic microworld and its smallest known
constituents will now be illustrated by the works of other Nobel Laureates.
It was realized, already in the first half of the 20th century, that such a further
journey into the microcosmos of new particles and interactions would also be
necessary for understanding the composition and evolution histories of the very
large structures of our universe, the "macrocosmos". At the present stage
elementary particle physics, astrophysics, and cosmology are strongly tied
together, as several examples presented here will show.
Another link connecting the smallest and the largest objects in our universe is
Albert Einstein's theories of relativity. Einstein first developed his special
theory of relativity in 1905, which expresses the mass-energy relationship E = m
c2. Then, in the next decade, he continued with his theory of general relativity,
which connects gravitational forces to the structure of space and time.
Calculations of effective masses for high energy particles, energy
transformations in radioactive decay as well as Dirac's predictions that
antiparticles may exist, are all based on his special theory of relativity. The
general theory is the basis for calculations of large scale motions in the
universe, including discussions of the properties of black holes. Einstein
received the Nobel Prize in Physics in 1921 (awarded in 1922), motivated by
work on the photo-electric effect which demonstrated the particle aspects of
light.
The works by Becquerel, the Curies, and Rutherford gave rise to new questions:
What was the source of energy in the radioactive nuclei that could sustain the
emission of α, β and γ radiation over very long time intervals, as observed for
some of them, and what were the heavy α particles and the nuclei themselves
actually composed of? The first of these problems (which seemed to violate the
law of conservation of energy, one of the most important principles of physics)
found its solution in the transmutation theory, formulated by Rutherford and
Frederick Soddy (Chemistry Prize for 1921, awarded in 1922). They followed
in detail several different series of radioactive decay and compared the energy
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emitted with the mass differences between "parent" and "daughter" nuclei. It
was also found that nuclei belonging to the same chemical element could have
different masses; such different species were called "isotopes". A Chemistry
Prize was given in 1922 to Francis W. Aston for his mass-spectroscopic
separation of a large number of isotopes of non-radioactive elements. Marie
Curie had by then already received a second Nobel Prize (this time in
Chemistry in 1911), for her discoveries of the chemical elements radium and
polonium.
All isotopic masses were found to be nearly equal to multiples of the mass of
the proton, a particle also first seen by Rutherford when he irradiated nitrogen
nuclei with α particles. But the different isotopes could not possibly be made up
entirely of protons since each particular chemical element must have one single
value for the total nuclear charge. Protons were actually found to make up less
than half of the nuclear mass, which meant that some neutral constituents had
to be present in the nuclei. James Chadwick first found conclusive evidence
for such particles, the neutrons, when he studied nuclear reactions in 1932. He
received the Physics Prize in 1935.
Soon after Chadwick's discovery, neutrons were put to work by Enrico Fermi
and others as a means to induce nuclear reactions that could produce new
"artificial" radioactivity. Fermi found that the probability of neutron-induced
reactions (which do not involve element transformations), increased when the
neutrons were slowed down and that this worked equally well for heavy
elements as for light ones, in contrast to charge-particle induced reactions. He
received the Physics Prize in 1938.
With neutrons and protons as the basic building blocks of atomic nuclei, the
branch of "nuclear physics" could be established and several of its major
achievements were distinguished by Nobel prizes. Ernest O. Lawrence, who
received the Physics Prize in 1939, built the first cyclotron in which acceleration
took place by successively adding small amounts of energy to particles
circulating in a magnetic field. With these machines, he was able to accelerate
charged nuclear particles to such high energies that they could induce nuclear
reactions and he obtained important new results. Sir John D. Cockcroft and
Ernest T.S. Walton instead, accelerated particles by direct application of very
high electrostatic voltages and were rewarded for their studies of transmutation
of elements in 1951.
Otto Stern received the Physics Prize in 1943 (awarded in 1944), for his
experimental methods of studying magnetic properties of nuclei, in particular
for measuring the magnetic moment of the proton itself. Isidor I. Rabi
increased the accuracy of magnetic moment determinations for nuclei by more
than two orders of magnitude, with his radio frequency resonance technique,
for which he was awarded the Physics Prize for 1944. Magnetic properties of
nuclei provide important information for understanding details in the build-up of
the nuclei from protons and neutrons. Later, in the second half of the century,
several theoreticians were rewarded for their work on the theoretical modeling
of this complex many-body system: Eugene P. Wigner (one-half of the prize),
Maria Goeppert-Mayer (one-fourth) and J. Hans D. Jensen (one-fourth) in
1963 and Aage N. Bohr, Ben R. Mottelson and L. James Rainwater in
1975. We will come back to these works under the heading "From Simple to
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Complex Systems".
As early as 1912, it was found by Victor F. Hess (awarded half the Prize in
1936 and the other half to Carl D. Anderson) that highly penetrating radiation is
also reaching us continuously from outer space. This "cosmic radiation" was first
detected by ionization chambers and later by Wilson's cloud chamber referred
to earlier. Properties of particles in the cosmic radiation could be inferred from
the curved particle tracks produced when a strong magnetic field was applied. It
was in this way that C. D. Anderson discovered the positron. Anderson and
Patrick M.S. Blackett showed that electron positron pairs could be produced
by γ rays (which needed a photon energy equal to at least 2mec2) and that
electrons and positrons could annihilate, producing γ rays as they disappeared.
Blackett received the Physics Prize in 1948 for his further development of the
cloud chamber and the discoveries made with it.
Although accelerators were further developed, cosmic radiation continued for a
couple of decades to be the main source of very energetic particles (and still
surpasses the most powerful accelerators on earth in this aspect, although with
extremely low intensities), and it provided the first glimpses of a completely
unknown subnuclear world. A new kind of particles, called mesons, was spotted
in 1937, having masses approximately 200 times that of electrons (but 10
times lighter than protons). In 1946, Cecil F. Powell clarified the situation by
showing that there were actually more than one kind of such particles present.
One of them, the "π meson", decays into the other one, the "µ meson". Powell
was awarded the Physics Prize in 1950.
By that time, theoreticians had already been speculating about the forces that
keep protons and neutrons together in nuclei. Hideki Yukawa suggested in
1935, that this "strong" force should be carried by an exchange particle, just as
the electromagnetic force was assumed to be carried by an exchange of virtual
photons in the new quantum field theory. Yukawa maintained that such a
particle must have a mass of about 200 electron masses in order to explain the
short range of the strong forces found in experiments. Powell's π meson was
found to have the right properties to act as a "Yukawa particle". The µ particle,
on the other hand, turned out to have a completely different character (and its
name was later changed from "µ meson" to "muon"). Yukawa received the
Physics Prize in 1949. Although later progress has shown that the strong force
mechanism is more complex than what Yukawa pictured it to be, he must still
be considered as the first one who led the thoughts on force carriers in this
fruitful direction.
More new particles were discovered in the 1950s, in cosmic radiation as well as
in collisions with accelerated particles. By the end of the 50s, accelerators could
reach energies of several GeV (109 electron volts) which meant that pairs of
particles, with masses equal to the proton mass, could be created by energy-tomass conversion. This was the method used by the team of Owen
Chamberlain and Emilio Segrè when they first identified and studied the
antiproton in 1955 (they shared the Physics Prize for 1959). High energy
accelerators also allowed more detailed studies of the structures of protons and
neutrons than before, and Robert Hofstadter was able to distinguish details of
the electromagnetic structure of the nucleons by observing how they scattered
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electrons of very high energy. He was rewarded with half the Physics Prize for
1961.
One after another, new mesons with their respective antiparticles appeared, as
tracks on photographic plates or in electronic particle detectors. The existence
of the "neutrino" predicted on theoretical grounds by Pauli already as early as
the 1930s, was established. The first direct experimental evidence for the
neutrino was provided by C. L. Cowan and Frederick Reines in 1957, but it
was not until 1995 that this discovery was awarded with one-half the Nobel
Prize (Cowan had died in 1984). The neutrino is a participant in processes
involving the "weak" interaction (such as β decay and π meson decay to muons)
and, as the intensity of particle beams increased, it became possible to produce
secondary beams of neutrinos from accelerators. Leon M. Lederman, Melvin
Schwartz and Jack Steinberger developed this method in the 1960s and
demonstrated that the neutrinos accompanying µ emission in π decay were not
identical to those associated with electrons in β decay; they were two different
particles, νµ and νe.
Physicists could now start to distinguish some order among the particles: the
electron (e), the muon (µ), the electron neutrino (νe), the muon neutrino (νµ)
and their antiparticles were found to belong to one class, called "leptons". They
did not interact by the "strong" nuclear force, which on the other hand,
characterized the protons, neutrons, mesons and hyperons (a set of particles
heavier than the protons). The lepton class was extended later in the 1970s
when Martin L. Perl and his team discovered the τ lepton, a heavier relative to
the electron and the muon. Perl shared the Physics Prize in 1995 with Reines.
All the leptons are still considered to be truly fundamental, i.e. point-like and
without internal structure, but for the protons, etc, this is no longer true.
Murray Gell-Mann and others managed to classify the strongly interacting
particles (called "hadrons") into groups with common relationships and ways of
interaction. Gell-Mann received the Physics Prize in 1969. His systematics was
based on the assumption that they were all built up from more elementary
constituents, called "quarks". The real proof that nucleons were built up from
quark-like objects came through the works of Jerome I. Friedman, Henry W.
Kendall and Richard E. Taylor. They "saw" hard grains inside these objects
when they studied how electrons (of still higher energy than Hofstadter could
use earlier) scattered inelastically on them. They shared the Physics Prize in
1990.
It was understood that all strongly interacting particles are built up by quarks.
In the middle of the 1970s a very short-lived particle, discovered independently
by the teams of Burton Richter and Samuel C.C. Ting, was found to contain
a so far, unknown type of quark which was given the name "charm". This quark
was a missing link in the systematics of the elementary particles and Burton
and Ting shared the Physics Prize in 1976. The present standard model of
particle physics sorts the particles into three families, with two quarks (and
their antiparticles) and two leptons in each: the "up" and "down" quarks, the
electron and the electron-neutrino in the first; the "strange" and the "charm"
quark, the muon and the muon neutrino in the second; the "top" and the
"bottom" quark, the tauon and the tau neutrino in the third. The force carriers
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for the combined electro-weak interaction are the photon, the Z-particle and the
W-bosons, and for the strong interaction between quarks the so-called gluons.
In 1983, the existence of the W- and Z-particles was proven by Carlo Rubbia's
team which used a new proton-antiproton collider with sufficient energy for
production of these very heavy particles. Rubbia shared the 1984 Physics Prize
with Simon van der Meer, who had made decisive contributions to the
construction of this collider by his invention of "stochastic cooling" of particles.
There are speculations that additional particles may be produced at energies
higher than those attainable with the present accelerators, but no experimental
evidence has been produced so far.
Cosmology is the science that deals with the structure and evolution of our
universe and the large-scale objects in it. Its models are based on the
properties of the known fundamental particles and their interactions as well as
the properties of space-time and gravitation. The "big-bang" model describes a
possible scenario for the early evolution of the universe. One of its predictions
was experimentally verified when Arno A. Penzias and Robert W. Wilson
discovered the cosmic microwave radiation background in 1960. They shared
one-half of the Physics Prize for 1978. This radiation is an afterglow of the
violent processes assumed to have occurred in the early stages of the big bang.
Its equilibrium temperature is 3 kelvin at the present age of the universe. It is
almost uniform when observed in different directions; the small deviations from
isotropy are now being investigated and will tell us more about the earliest
history of our universe.
Outer space has been likened to a large arena for particle interactions where
extreme conditions, not attainable in a laboratory, are spontaneously created.
Particles may be accelerated to higher energies than in any accelerator on
earth, nuclear fusion reactions proliferate in the interior of stars, and gravitation
can compress particle systems to extremely high densities. Hans A. Bethe first
described the hydrogen and carbon cycles, in which energy is liberated in stars
by the fusion of protons into helium nuclei. For this achievement he received
the Physics Prize in 1967.
Subramanyan Chandrasekhar described theoretically the evolution of stars,
in particular those ending up as "white dwarfs". Under certain conditions the
end product may also be a "neutron star", an extremely compact object, where
all protons have been converted into neutrons. In supernova explosions, the
heavy elements created during stellar evolution are spread out into space. The
details of some of the most important nuclear reactions in stars and heavy
element formation were elucidated by William A. Fowler both in theory and in
experiments using accelerators. Fowler and Chandrasekhar received one-half
each of the 1983 Physics Prize.
Visible light and cosmic background radiation are not the only forms of
electromagnetic waves that reach us from outer space. At longer wavelengths,
radio astronomy provides information on astronomical objects not obtainable by
optical spectroscopy. Sir Martin Ryle developed the method where signals
from several separated telescopes are combined in order to increase the
resolution in the radio source maps of the sky. Antony Hewish and his group
made an unexpected discovery in 1964 using Ryle's telescopes: radio frequency
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pulses were emitted with very well-defined repetition rates by some unknown
objects called pulsars. These were soon identified as neutron stars, acting like
fast rotating lighthouses emitting radiowaves because they are also strong
magnets. Ryle and Hewish shared the Physics Prize in 1974.
By 1974, pulsar search was already routine among radio astronomers, but a
new surprise came in the summer of the same year when Russell A. Hulse
and Joseph H. Taylor, Jr. noticed periodic modulations in the pulse
frequencies of a newly discovered pulsar, called PSR 1913+16. It was the first
double pulsar detected, so named because the emitting neutron star happened
to be one of the components of a close double star system, with the other
component of about equal size. This system has provided, by observation over
more than 20 years, the first concrete evidence for gravitational radiation. The
decrease of its rotational frequency is in close agreement with the predictions
based on Einstein's theory, for losses caused by this kind of radiation. Hulse and
Taylor shared the Physics Prize in 1993. However, the direct detection of
gravitational radiation on earth still has to be made.
From Simple to Complex Systems
If all the properties of the elementary particles as well as the forces that may
act between them were known in every detail, would it then be possible to
predict the behavior of all systems composed of such particles? The search for
the ultimate building blocks of nature and of the proper theoretical description
of their interactions (on the macro as well as the micro scale), has partly been
motivated by such a reductionistic program. All scientists would not agree that
such a synthesis is possible even in principle. But even if it were true, the
calculations of complex system behavior would very soon be impossible to
handle when the number of particles and interactions in the system is
increased. Complex multi-particle systems are therefore described in terms of
simplified models, where only the most essential features of their particle
compositions and interactions are used as starting points. Quite often, it is
observed that complex systems develop features called "emergent properties",
not straightforwardly predictable from the basic interactions between their
constituents.
Atomic Nuclei
The first complex systems from the reductionist's point of view are the
nucleons, i.e. neutrons and protons composed of quarks and gluons. The
second is the atomic nuclei, which to a first approximation are composed of
separate nucleons. The first advanced model of nuclear structure was the
nuclear shell model, put up by the end of the 1940s by Maria Goeppert-Mayer
and Johannes D. Jensen who realized that at least for nuclei with nearly
spherical shape, the outer nucleons fill up energy levels like electrons in atoms.
However, the order is different; it is determined by another common potential
and by the specific strong spin-orbit coupling of the nuclear forces. Their model
explains why nuclei with so-called "magic numbers" of protons or neutrons are
particularly stable. They shared the Physics Prize in 1963 together with Eugene
Wigner, who had formulated fundamental symmetry principles important in
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both nuclear and particle physics.
Nuclei with nucleon numbers far from the magic ones are not spherical. Niels
Bohr had already worked with a liquid drop model for such deformed nuclei
which may take ellipsoidal shapes, and in 1939 it was found that excitation of
certain strongly deformed nuclei could lead to nuclear fission, i.e. the breakup
of such nuclei into two heavy fragments. Otto Hahn received the Chemistry
Prize in 1944 (awarded in 1945) for the discovery of this new process. The nonspherical shape of deformed nuclei allows new collective, rotational degrees of
freedom, as do also the collective vibrations of nucleons. Models describing such
excitations of the nuclei were developed by James Rainwater, Aage Bohr (son of
Niels Bohr) and Ben Mottelson, who jointly received the Physics Prize in 1975.
The nuclear models mentioned above, were based not only on general, guiding
principles, but also on the steadily increasing information from nuclear
spectroscopy. Harold C. Urey discovered deuterium, a heavy isotope of
hydrogen, for which he was awarded the Chemistry Prize in 1934. Fermi,
Lawrence, Cockcroft, and Walton mentioned in the previous section developed
methods for the production of unstable nuclear isotopes. For their extension of
the nuclear isotope chart to the heaviest elements, Edwin M. McMillan and
Glenn T. Seaborg were awarded, again with a Chemistry Prize (in 1951). In
1954, Walther Bothe received one-half of the Physics Prize and the other half
was awarded to Max Born, mentioned earlier. Bothe developed the coincidence
method, which allowed spectroscopists to select generically related sequences
of nuclear radiation from the decay of nuclei. This turned out to be important,
particularly for the study of excited states of nuclei and their electromagnetic
properties.
Atoms
The electronic shells of the atoms, when considered as many-body systems, are
easier to handle than the nuclei (which actually contain not only protons and
neutrons but also more of other, short-lived "virtual" particles than the atoms).
This is due to the weakness and simplicity of the electromagnetic forces as
compared to the "strong" forces holding the nuclei together. With the quantum
mechanics developed by Schrödinger, Heisenberg, and Pauli, and the relativistic
extensions by Dirac, the main properties of the atomic electrons could be
reasonably well described. However, a long standing problem has remained,
namely to solve the mathematical problems connected with the mutual
interactions between the electrons after the dominating attraction by the
positive nuclei has been taken into account. One aspect of this was addressed in
the work by one of the most recent Chemistry Laureates (1998), Walter Kohn.
He developed the "density functional" method which is applicable to free atoms
as well as to electrons in molecules and solids.
At the beginning of the 20th century, the periodic table of elements was not yet
complete. The early history of the Nobel Prizes includes the discoveries of some
of the then missing elements. Lord Raleigh (John William Strutt) noticed
anomalies in the relative atomic masses when oxygen and nitrogen samples
were taken directly from the air that surrounds us, instead of separating them
from chemical compounds. He concluded that the atmosphere must contain a
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so far unknown constituent, which was the element argon with atomic mass 20.
He was awarded the Physics Prize in 1904, the same year that Sir William
Ramsay obtained the Chemistry Prize for isolating the element helium.
In the second half of the 20th century, there has been a spectacular
development of atomic spectroscopy and the precision by which one can
measure the transitions between atomic or molecular states that fall in the
microwave and optical range. Alfred Kastler (who received the Physics Prize in
1966) and his co-workers showed in the 1950s that electrons in atoms can be
put into selected excited substates by the use of polarized light. After radiative
decay, this can also lead to an orientation of the spins of ground-state atoms.
The subsequent induction of radio frequencey transitions opened possibilities to
measure properties of the quantized states of electrons in atoms in much
greater detail than before. A parallel line of development led to the invention of
masers and lasers, which are based on the "amplification of stimulated emission
of radiation" in strong microwave and optical (light) fields, respectively (effects
which in principle would have been predictable from Einstein's equations
formulated in 1917 but were not discussed in practical terms until early in the
1950s).
Charles H. Townes developed the first maser in 1958. Theoretical work on the
maser principle was made by Nikolay G. Basov and Aleksandr M.
Prokhorov. The first maser used a stimulated transition in the ammonia
molecule. It emitted an intense microwave radiation, which unlike that of
natural emitters, was coherent (i.e. with all photons in phase). Its frequency
sharpness soon made it an important tool in technology, for time-keeping and
other purposes. Townes received half the Physics Prize for 1964 and Basov and
Prokhorov shared the other half.
For radiation in the optical range, lasers were later developed in several
laboratories. Nicolaas Bloembergen and Arthur L. Schawlow were
distinguished in 1981 for their work on precision laser spectroscopies of atoms
and molecules. The other half of that year's prize was awarded to Kai M.
Siegbahn (son of Manne Siegbahn), who developed another high-precision
method for atomic and molecular spectroscopy based on electrons emitted from
inner electron shells when hit by X-rays with very well-defined energy. His
photo- and Auger-electron spectroscopy is used as an analytical tool in several
other areas of physics and chemistry.
The controlled interplay between atomic electrons and electromagnetic fields
has continued to provide ever more detailed information about the structure of
electronic states in atoms. Norman F. Ramsey developed precision methods
based on the response to external radio frequency signals by free atoms in
atomic beams and Wolfgang Paul invented atomic "traps", built by
combinations of electric and magnetic fields acting over the sample volumes.
Hans G. Dehmelt's group was the first to isolate single particles (positrons) as
well as single atoms in such traps. For the first time, experimenters could
"communicate" with individual atoms by microwave and laser signals. This
enabled the study of new aspects of quantum mechanical behavior as well as
further increased precision in atomic properties and the setting of time
standards. Paul and Dehmelt received the 1989 Physics Prize and the other half
was awarded to Ramsey.
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The latest step in this development has involved the slowing down of the
motion of atoms in traps to such an extent that it would correspond to microkelvin temperatures, had they been in thermal equilibrium in a gas. This is done
by exposing them to "laser cooling" through a set of ingenious schemes
designed and carried out in practice by Steven Chu, Claude CohenTannoudji and William D. Phillips, whose research groups manipulated
atoms by collisions with laser photons. Their work, which was recognized by the
1997 Physics Prize, promises important applications in general measurement
technology in addition to a still more increased precision in the determination of
atomic quantities.
Molecules and Plasmas
Molecules are composed of atoms. They form the next level of complexity when
considered as many-body systems. But molecular phenomena have traditionally
been viewed as a branch of chemistry (as exemplified by the Chemistry Prize in
1936 to Petrus J.W. Debye), and have only rarely been in the focus for Nobel
Prizes in Physics. One exception is the recognition of the work by Johannes
Diderik van der Waals, who formulated an equation of state for molecules in
a gas taking into account the mutual interaction between the molecules as well
as the reduction of the free volume due to their finite size. van der Waals'
equation has been an important starting point for the description of the
condensation of gases into liquids. He received the 1910 Physics Prize. Jean B.
Perrin studied the motion of small particles suspended in water and received
the 1926 Physics Prize. His studies allowed a confirmation of Einstein's
statistical theory of Brownian motion as well as of the laws governing the
equilibrium of suspended particles under the influence of gravity.
In 1930, Sir C. Venkata Raman received the Physics Prize for his observations
that light scattered from molecules contained components which were shifted in
frequency with respect to the infalling monochromatic light. These shifts are
caused by the molecules' gain or loss of characteristic amounts of energy when
they change their rotational or vibrational motion. Raman spectroscopy soon
became an important source of information on molecular structure and
dynamics.
A plasma is a gaseous state of matter in which the atoms or molecules are
strongly ionized. Mutual electromagnetic forces, both between the positive ions
themselves and between the ions and the free electrons, are then playing
dominant roles, which adds to the complexity as compared to the situation in
neutral atomic or molecular gases. Hannes Alfvén demonstrated in the 1940s
that a new type of collective motion, called "magneto-hydrodynamical waves"
can arise in such systems. These waves play a crucial role for the behavior of
plasmas, in the laboratory as well as in the earth's atmosphere and in cosmos.
Alfvén received half of the 1970 Physics Prize.
Condensed Matter
Crystals are characterized by a regular arrangement of atoms. Relatively soon
after the discovery of the X-rays, it was realized by Max von Laue that such
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rays were diffracted when passing through crystalline solids, like light passing
an optical grating. This effect is related to the fact that the wavelength of
common X-ray sources happens to coincide with typical distances between
atoms in these materials. It was first used systematically by Sir William Henry
Bragg and William Lawrence Bragg (father and son) to measure interatomic
distances and to analyse the geometrical arrangement of atoms in simple
crystals. For their pioneering work on X-ray crystallography (which has later
been developed to a high degree of sophistication), they received the Nobel
Prize in Physics; Laue in 1914 and the Braggs in 1915.
The crystalline structure is the most stable of the different ways in which atoms
can be organized to form a certain solid at the prevalent temperature and
pressure conditions. In the 1930s Percy W. Bridgman invented devices by
which very high pressures could be applied to different solid materials and
studied changes in their crystalline, electric, magnetic and thermal properties.
Many crystals undergo phase transitions under such extreme circumstances,
with abrupt changes in the geometrical arrangements of their atoms at certain
well-defined pressures. Bridgman received the Physics Prize in 1946 for his
discoveries in the field of high pressure physics.
Low-energy neutrons became available in large numbers to the experimenters
through the development of fission reactors in the 1940s. It was found that
these neutrons, like X-rays, were useful for crystal structure determinations
because their associated de Broglie wavelengths also fall in the range of typical
interatomic distances in solids. Clifford G. Shull contributed strongly to the
development of the neutron diffraction technique for crystal structure
determination, and showed also that the regular arrangement of magnetic
moments on atoms in ordered magnetic materials can give rise to neutron
diffraction patterns, providing a new powerful tool for magnetic structure
determination.
Shull was rewarded with the Physics Prize in 1994, together with Bertram N.
Brockhouse, who specialized in another aspect of neutron scattering on
condensed material: the small energy losses resulting when neutrons excite
vibrational modes (phonons) in a crystalline lattice. For this purpose,
Brockhouse developed the 3-axis neutron spectrometer, by which complete
dispersion curves (phonon energies as function of wave vectors) could be
obtained. Similar curves could be recorded for vibrations in magnetic lattices
(the magnon modes).
John H. Van Vleck made significant contributions to the theory of magnetism
in condensed matter in the years following the creation of quantum mechanics.
He calculated the effects of chemical binding on the paramagnetic atoms and
explained the effects of temperature and applied magnetic fields on their
magnetism. In particular, he developed the theory of crystal field effects on the
magnetism of transition metal compounds, which has been of great importance
for understanding the function of active centers in compounds for laser physics
as well as in biomolecules. He shared the Physics Prize in 1977 with Philip W.
Anderson and Sir Nevill F. Mott (see below).
Magnetic atoms can have their moments all ordered in the same direction in
each domain (ferromagnetism), with alternating "up" and "down" moments of
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the same size (simple antiferromagnets) or with more complicated patterns
including different magnetic sublattices (ferrimagnets, etc). Louis E.F. Néel
introduced basic models to describe antiferromagnetic and ferrimagnetic
materials, which are important components in many solid state devices. They
have been extensively studied by the aforementioned neutron diffraction
techniques. Néel obtained one-half of the Physics Prize in 1970.
The geometric ordering of atoms in crystalline solids as well as the different
kinds of magnetic order, are examples of general ordering phenomena in nature
when systems find an energetically favorable arrangement by choosing a
certain state of symmetry. The critical phenomena, which occur when
transitions between states of different symmetry are approached (for instance
when temperature is changed), have a high degree of universality for different
types of transitions, including the magnetic ones. Kenneth G. Wilson, who
received the Physics Prize in 1982, developed the so-called renormalization
theory for critical phenomena in connection with phase transitions, a theory
which has also found application in certain field theories of particle physics.
Liquid crystals form a specific class of materials that show many interesting
features, from the point of view of fundamental interactions in condensed
matter as well as for technical applications. Pierre-Gilles de Gennes
developed the theory for the behavior of liquid crystals and their transitions
between different ordered phases (nematic, smectic, etc). He used also
statistical mechanics to describe the arrangements and dynamics of polymer
chains, thereby showing that methods developed for ordering phenomena in
simple systems can be generalized to the complex ones occurring in "soft
condensed matter". For this, he received the Physics Prize in 1991.
Another specific form of liquid that has received attention is liquid helium. At
normal pressures, this substance remains liquid down to the lowest
temperatures attainable. It also shows large isotope effects, since 4He
econdenses to liquid at 4.2 K, while the more rare isotope 3He remains in
gaseous form down to 3.2 K. Helium was first liquefied by Heike KamerlinghOnnes in 1909. He received the Physics Prize in 1913 for the production of
liquid helium and for his investigations of properties of matter at low
temperatures. Lev D. Landau formulated fundamental concepts (e.g. the
"Landau liquid") concerning many-body effects in condensed matter and applied
them to the theory of liquid helium, explaining specific phenomena occuring in
4
He such as the superfluidity (see below), the "roton" excitations, and certain
acoustic phenomena. He was awarded the Physics Prize in 1962.
Several of the experimental techniques used for the production and study of low
temperature phenomena were developed by Pyotr L. Kapitsa in the 1920s and
30s. He studied many aspects of liquid 4He and showed that it was superfluid
(i.e. flowing without friction) below 2.2 K. The superfluid state was later
understood to be a manifestation of macroscopic quantum coherence in a BoseEinstein type of condensate (theoretically predicted in 1920) with many features
in common with the superconducting state for electrons in certain conductors.
Kapitsa received one-half of the Physics Prize for 1978.
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In liquid 3He, additional, unique phenomena show up because each He nucleus
has a non-zero spin in contrast to those of 4He. Thus, it is a fermion type of
particle, and should not be able to participate in Bose-Einstein condensation,
which works only for bosons. However, like in superconductivity (see below)
pairs of spin-half particles can form "quasi-bosons" that can condense into a
superfluid phase. Superfluidity in 3He, whose transition temperature is reduced
by a factor of a thousand compared to that of liquid 4He, was discovered by
David M. Lee, Douglas D. Osheroff and Robert C. Richardson, who
received the Physics Prize in 1996. They observed three different superfluid
phases, showing complex vortex structures and interesting quantum behavior.
Electrons in condensed matter can be localized to their respective atoms as in
insulators, or they can be free to move between atomic sites, as in conductors
and semiconductors. In the beginning of the 20th century, it was known that
metals emitted electrons when heated to high temperatures, but it was not
clear whether this was due only to thermal excitation of the electrons or if
chemical interactions with the surrounding gas were also involved. Through
experiments carried out in high vacuum, Owen W. Richardson could finally
establish that electron emission is a purely thermionic effect and a law based on
the velocity distribution of electrons in the metal could be formulated. For this,
Richardson received the Physics Prize in 1928 (awarded in 1929.)
The electronic structure determines the electric, magnetic, and optical
properties of solids and is also of major importance for their mechanical and
thermal behavior. It has been one of the major tasks of physicists in the 20th
century to measure the states and dynamics of electrons and model their
behavior so as to understand how they organize themselves in various types of
solids. It is natural that the most unexpected and extreme manifestations of
electron behavior have attracted the strongest interest in the community of
solid state physicists. This is also reflected in the Nobel Prize in Physics: several
prizes have been awarded for discoveries connected with superconductivity and
for some of the very specific effects displayed in certain semiconducting
materials.
Superconductivity was discovered as early as 1911 by Kamerlingh-Onnes, who
noticed that the electrical resistivity of mercury dropped to less than one
billionth of its ordinary value when it was cooled well below a transition
temperature of TC, which is about 4 K. As mentioned earlier, he received the
Physics Prize in 1913. However, it would take a very long period of time before
it was understood why electrons could flow without resistance in certain
conductors at low temperature. But in the beginning of the 1960s Leon N.
Cooper, John Bardeen and J. Robert Schrieffer formulated a theory based
on the idea that pairs of electrons (with opposite spins and directions of motion)
can lower their energy by an amount E by sharing exactly the same
g
deformation of the crystalline lattice as they move. Such "Cooper pairs" act as
bosonic particles. This allows them to move as a coherent macroscopic fluid,
undisturbed as long as the thermal excitations (of energy kT) are lower in
energy than the energy E gained by the pair formation. The so-called BCSg
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theory was rewarded with the Physics Prize in 1972.
This breakthrough in the understanding of the quantum mechanical basis led to
further progress in superconducting circuits and components: Brian D.
Josephson analysed the transfer of superconducting carriers between two
superconducting metals, separated by a very thin layer of normal-conducting
material. He found that the quantum phase, which determines the transport
properties, is an oscillating function of the voltage applied over this kind of
junction. The Josephson effect has important applications in precision
measurements, since it establishes a relation between voltage and frequency
scales. Josephson received one-half of the Physics Prize for 1973. Ivar
Giaever, who invented and studied the detailed properties of the "tunnel
junction", an electronic component based on superconductivity, shared the
second half with Leo Esaki for work on tunneling phenomena in
semiconductors (see below).
Although a considerable number of new superconducting alloys and compounds
were discovered over the first 75 years that followed Kamerlingh-Onnes'
discovery, it seemed as if superconductivity would forever remain a typical low
temperature phenomenon, with the limit for transition temperatures slightly
above 20 K. It therefore came as a total surprise when J. Georg Bednorz and
K. Alexander Müller showed that a lanthanum-copper oxide could be made
superconducting up to 35 K by doping it with small amounts of barium. Soon
thereafter, other laboratories reported that cuprates of similar structure were
superconducting up to about 100 K. This discovery of "high temperature
superconductors" triggered one of the greatest efforts in modern physics: to
understand the basic mechanism for superconductivity in these extraordinary
materials. Bednorz and Müller shared the Physics Prize in 1987.
Electron motion in the normal conducting state of metals has been modeled
theoretically with increasing degree of sophistication ever since the advent of
quantum mechanics. One of the early major steps was the introduction of the
Bloch wave concept, named after Felix Bloch (half of the Physic Prize for
magnetic resonance in 1952). Another important concept, "the electron fluid" in
conductors, was introduced by Lev Landau (see liquid He). Philip W. Anderson
made several important contributions to the theory of electronic structures in
metallic systems, in particular concerning the effects of inhomogeneities in
alloys and magnetic impurity atoms in metals. Nevill F. Mott worked on the
general conditions for electron conductivity in solids and formulated rules for
the point at which an insulator becomes a conductor (the Mott transition) when
composition or external parameters are changed. Anderson and Mott shared the
1977 Physics Prize with John H. Van Vleck for their theoretical investigations of
the electronic structure of magnetic and disordered systems.
An early Physics Prize (1920) was given to Charles E. Guillaume for his
discovery that the thermal expansion of certain nickel steels, so-called "invar"
alloys, was practically zero. This prize was mainly motivated by the importance
of these alloys for precision measurements in physics and geodesy, in particular
when referring to the standard meter in Paris. The invar alloys have been
extensively used in all kinds of high-precision mechanical devices, watches, etc.
The theoretical background for this temperature independence has been
explained only recently. Also very recently (1998), Walter Kohn was
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recognized by a Nobel Prize in Chemistry for his methods of treating quantum
exchange correlations, by which important limitations for the predictive power
of electronic structure calculations, in solids as well as molecules, have been
overcome.
In semiconductors, electron mobility is strongly reduced because there are
forbidden regions for the energy of the electrons that take part in conduction,
the "energy gaps". It was only after the basic roles of doping of ultra-pure
silicon (and later other semiconducting materials) with chosen electrondonating or electron-accepting agents were understood, that semiconductors
could be used as components in electronic engineering. William B. Shockley,
John Bardeen (see also BCS-theory) and Walter H. Brattain carried out
fundamental investigations of semiconductors and developed the first transistor.
This was the beginning of the era of "solid state electronics". They shared the
Physics Prize in 1956.
Later, Leo Esaki developed the tunnel diode, an electronic component that has
a negative differential resistance, a technically interesting property. It is
composed of two heavily n and p doped semiconductors, that have an excess of
electrons on one side of the junction and a deficit on the other. The tunneling
effect occurs at bias voltages larger than the gap in the semi-conductors. He
shared the Physics Prize for 1973 with Brian D. Josephson.
With modern techniques it is possible to build up well-defined, thin-layered
structures of different semiconducting materials, in direct contact with each
other. With such "heterostructures" one is not limited to the band-gaps
provided by semi-conducting materials like silicon and germanium. Herbert
Kroemer analysed theoretically the mobility of electrons and holes in
heterostructure junctions. His propositions led to the build up of transistors with
much improved characteristics, later called HEMTs (high electron mobility
transistors), which are very important in today's high-speed electronics.
Kroemer suggested also, at about the same time as Zhores I. Alferov, the use
of double heterostructures to provide conditions for laser action. Alferov later
built the first working pulsed semiconductor laser in 1970. This marked the
beginning of the era of modern optoelectronic devices now used in laser diodes,
CD-players, bar code readers and fiber optics communication. Alferov and
Kroemer recently shared one-half of the Physics Prize for the year 2000. The
other half went to Jack S. Kilby, co-inventor of the integrated circuit (see the
next section on Physics and Technology).
By applying proper electrode voltages to such systems one can form "inversion
layers", where charge carriers move essentially only in two dimensions. Such
layers have turned out to have some quite unexpected and interesting
properties. In 1982, Klaus von Klitzing discovered the quantized Hall effect.
When a strong magnetic field is applied perpendicularly to the plane of a quasi
two-dimensional layer, the quantum conditions are such that an increase of
magnetic field does not give rise to a linear increase of voltage on the edges of
the sample, but a step-wise one. Between these steps, the Hall resistance is
h/ie2, where i's are integers corresponding to the quantized electron orbits in the
plane. Since this provides a possibility to measure the ratio between two
fundamental constants very exactly, it has important consequences for
measurement technology. von Klitzing received the Physics Prize in 1985.
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A further surprise came shortly afterwards when Daniel C. Tsui and Horst L.
Störmer made refined studies of the quantum Hall effect using inversion layers
in materials of ultra-high purity. Plateaus appeared in the Hall effect not only
for magnetic fields corresponding to the filling of orbits with one, two, three,
etc, electron charges, but also for fields corresponding to fractional charges!
This could be understood only in terms of a new kind of quantum fluid, where
the motion of independent electrons of charge e is replaced by excitations in a
multi-particle system which behave (in a strong magnetic field) as if charges of
e/3, e/5, etc were involved. Robert B. Laughlin developed the theory that
describes this new state of matter and shared the 1998 Physics Prize with Tsui
and Störmer.
Sometimes, discoveries made in one field of physics turn out to have important
applications in quite different areas. One example, of relevance for solid state
physics, is the observation by Rudolf L. Mössbauer in the late 50s, that nuclei
in "absorber" atoms can be resonantly excited by γ rays from suitably chosen
"emitter" atoms, if the atoms in both cases are bound in such a way that recoils
are eliminated. The quantized energies of the nuclei in the internal electric and
magnetic fields of the solid can be measured since they correspond to different
positions of the resonances, which are extremely sharp. This turned out to be
important for the determination of electronic and magnetic structure of many
substances and Mössbauer received half the Physics Prize in 1961 and R.
Hofstadter the other half.
Physics and Technology
Many of the discoveries and theories mentioned so far in this survey have had
an impact on the development of technical devices; by opening completely new
fields of physics or by providing ideas upon which such devices can be built.
Conspicuous examples are the works of Shockley, Bardeen, and Brattain which
led to the transistors and started a revolution in electronics, and the basic
research by Townes, Basov, and Prokhorov which led to the development of
masers and lasers. It could also be mentioned that particle accelerators are now
important tools in several areas of materials science and in medicine. Other
works honored by Nobel Prizes have had a more direct technical motivation, or
have turned out to be of particular importance for the construction of devices
for the development of communication and information.
An early Physics Prize (1912) was given to Nils Gustaf Dalén for his invention
of an automatic "sun-valve", extensively used for lighting beacons and light
buoys. It was based on the difference in radiation of heat from reflecting and
black bodies: one out of three parallel bars in his device was blackened, which
gave rise to a difference in heat absorption and length expansion of the bars
during sunshine hours. This effect was used to automatically switch off the gas
supply in daytime, eliminating much of the need for maintenance at sea.
Optical instrumentation and techniques have been the topics for prizes at
several occasions. Around the turn of the century, Gabriel Lippmann
developed a method for color photography using interference of light. A mirror
was placed in contact with the emulsion of a photographic plate in such a way
that when it was illuminated, reflection in the mirror gave rise to standing
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waves in the emulsion. Developing resulted in a stratification of the grains of
silver and when such a plate was looked at in a mirror, the picture was
reproduced in its natural colors. The Physics Prize in 1908 was awarded to
Lippmann. Unfortunately, Lippmann's method requires very long exposure
times. It has later been superseded by other techniques for photography but
has found new applications in high-quality holograms.
In optical microscopy it was shown by Frits Zernike that even very weakly
absorbing (virtually transparent) objects can be made visible if they consist of
regions with different refractive indices. In Zernike's "phase-contrast
microscope" it is possible to distinguish patches of light that have undergone
different phase changes caused by this kind of inhomogeneity. This microscope
has been of particular importance for observing details in biological samples.
Zernike received the Physics Prize in 1953. In the 1940s, Dennis Gabor laid
down the principles of holography. He predicted that if an incident beam of light
is allowed to interfere with radiation reflected from a two-dimensional array of
points in space, it would be possible to reproduce a three-dimensional picture of
an object. However, the realization of this idea had to await the invention of
lasers, which could provide the coherent light necessary for such interference
phenomena to be observed. Gabor was awarded the Physics Prize in 1971.
Electron microscopy has had an enormous impact on many fields of natural
sciences. Soon after the wave nature of electrons was clarified by C. J. Davisson
and G. P. Thomson, it was realized that the short wavelengths of high energy
electrons would make possible a much increased magnification and resolution
as compared to optical microscopes. Ernst Ruska made fundamental studies in
electron optics and designed the first working electron microscope early in the
1930s. However, it would take more than 50 years before this was recognized
by a Nobel Prize.
Ruska obtained half of the Physics Prize for 1986, while the other half was
shared between Gerd Binnig and Heinrich Rohrer, who had developed a
completely different way to obtain pictures with extremely high resolution. Their
method is applicable to surfaces of solids and is based on the tunneling of
electrons from very thin metallic tips to atoms on the surface when the tip is
moved at very close distance to it (about 1 nm). By keeping the tunneling
current constant a moving tip can be made to follow the topography of the
surface, and pictures are obtained by scanning over the area of interest. By this
method, single atoms on surfaces can be visualized.
Radio communication is one of the great technical achievements of the 20th
century. Guglielmo Marconi experimented in the 1890s with the newly
discovered Hertzian waves. He was the first one to connect one of the terminals
of the oscillator to the ground and the other one to a high vertical wire, the
"antenna", with a similar arrangement at the receiving station. While Hertz'
original experiments were made within a laboratory, Marconi could extend
signal transmission to distances of several kilometers. Further improvement
was made by Carl Ferdinand Braun (also father of the "Braunian tube", an
early cathode ray oscilloscope), who introduced resonant circuits in the Hertzian
oscillators. The tunability and the possibility to produce relatively undamped
outgoing oscillations greatly increased the transmission range, and in 1901
Marconi succeeded in establishing radio connection across the Atlantic. Marconi
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and Braun shared the 1909 Nobel Prize in Physics.
At this stage, it was not understood how radio waves could reach distant places
(practically "on the other side of the earth"), keeping in mind that they were
known to be of the same nature as light, which propagates in straight lines in
free space. Sir Edward V. Appleton finally proved experimentally that an
earlier suggestion by Heaviside and Kennelly, that radio waves were reflected
between different layers with different conductance in the atmosphere, was the
correct explanation. Appleton measured the interference of the direct and
reflected waves at various wavelengths and could determine the height of
Heaviside's layer; in addition he found another one at a higher level which still
bears his name. Appleton received the Physics Prize in 1947.
Progress in nuclear and particle physics has always been strongly dependent on
advanced technology (and sometimes a driving force behind it). This was
already illustrated in connection with the works of Cockcroft and Walton and of
Lawrence, who developed linear electrostatic accelerators and cyclotrons,
respectively. Detection of high energy particles is also a technological challenge,
the success of which has been recognized by several Nobel Prizes.
The Physics Prize in 1958 was jointly awarded to Pavel A. Cherenkov, Il'ja M.
Frank and Igor Y. Tamm for their discovery and interpretation of the
Cherenkov effect. This is the emission of light, within a cone of specific opening
angle around the path of a charged particle, when its velocity exceeds the
velocity of light in the medium in which it moves. Since this cone angle can be
used to determine the velocity of the particle, the work by these three
physicists soon became the basis for fruitful detector developments.
The visualization of the paths of particles taking part in reactions is necessary
for the correct interpretation of events occurring at high energies. Early
experiments at relatively low energies used the tracks left in photographic
emulsions. Charles T.R. Wilson developed a chamber in which particles were
made visible by the fact that they leave tracks of ionized gas behind them. In
the Wilson chamber the gas is made to expand suddenly, which lowers the
temperature and leads to condensation of vapour around the ionized spots;
these drops are then photographed in strong light. Wilson received half of the
Physics Prize in 1927, the other half was awarded to Arthur H. Compton.
A further step in the same direction came much later when Donald A. Glaser
invented the "bubble chamber". In the 1950s accelerators had reached energies
of 20-30 GeV and earlier methods were inadequate; for the Wilson chamber the
path lengths in the gas would have been excessive. The atomic nuclei in a
bubble chamber (usually containing liquid hydrogen) are used as targets, and
the tracks of produced particles can be followed. At the temperature of
operation the liquid is superheated and any discontinuity, like an ionized region,
immediately leads to the formation of small bubbles. Essential improvements
were made by Luis W. Alvarez, in particular concerning recording techniques
and data analysis. His work contributed to a fast extension of the number of
known elementary particles then known, in particular the so-called
"resonances" (which were later understood as excited states of systems
composed of quarks and gluons). Glaser received the Physics Prize in 1960 and
Alvarez in 1968.
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Bubble chambers were, up to the end of the 80s, the work horses of all high
energy physics laboratories but have later been superseded by electronic
detection systems. The latest step in detector development recognized by a
Nobel Prize (in 1992) is the work of Georges Charpak. He studied in detail the
ionization processes in gases and invented the "wire chamber", a gas-filled
detector where densely spaced wires pick up electric signals near the points of
ionization, by which the paths of particles can be followed. The wire chamber
and its followers, the time projection chamber and several large wire
chamber/scintillator/Cherenkov detector arrangements, combined into
complex systems, has made possible the selective search for extremely rare
events (like heavy quark production), which are hidden in strong backgrounds
of other signals.
The first Nobel Prize (year 2000) in the new millennium was awarded in half to
Jack S. Kilby for achievements that laid the foundations for the present
information technology. In 1958, he fabricated the first integrated circuit where
all electronic components are built on one single block of semiconducting
material, later called "chip". This opened the way for miniaturization and mass
production of electronic circuits. In combination with the development of
components based on heterostructures described in an earlier section (for which
Alferov and Kroemer shared the other half of the Prize), this has led to the "ITrevolution" that has reshaped so much our present society.
Further Remarks
In reading the present survey, it should be kept in mind that the number of
Nobel awards is limited (according to the present rules, at most 3 persons can
share a Nobel Prize each year). So far, 163 laureates have received Nobel
Prizes for achievements in physics. Often, during the selection process,
committees have had to leave out several other important, "near Nobel-worthy"
contributions. For obvious reasons, it has not been possible to mention any of
these other names and contributions in this survey. Still, the very fact that a
relatively coherent account of the development of physics can be formulated,
hinging as here on the ideas and experiments made by Nobel Laureates, can be
taken as a testimony that most of the essential features in this fascinating
journey towards an understanding of the world we inhabit have been covered
by the Nobel Prizes in Physics.
*Now published as a chapter of the book: "The Nobel Prize: The First 100
Years", Agneta Wallin Levinovitz and Nils Ringertz, eds., Imperial College Press
and World Scientific Publishing Co. Pte. Ltd., 2001.
Reference: http://nobelprize.org/cgi-bin/print?
from=/nobel_prizes/physics/articles/karlsson/index.html
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